The electron microscope continues to provide important support to the research program of the Epidemiology Section of LID focussed on the study of enteric diseases. Past highlights of these investigations have included inquiry into the etiology of diseases of unknown cause and studies of fastidious agents of disease that defied cultivation in any cell culture system. The electron microscope, as a unique instrument played a major role in the discoveries of the 27nm Norwalk virus and the establishment of its etiologic role in human diarrheal disease and of the 27nm hepatitis A virus particle and establishment of its etiologic role. Although intensive efforts have been made by numerous groups of investigators, the Norwalk and related human enteric caliciviruses have yet to be cultivated in any tissue culture system. This, along with the recent expression of 27nm virus-like particles of some members of this group of agents, as well as studies of expressed viral proteins of rotavirus, have contributed to the continuing important role of the electron microscope in the research program of the Epidemiology Section. The electron microscope continues to be the only method (i) for direct detection of the noroviruses and derivative 27nm expressed virus-like particles and (ii) for unraveling the antigenic relationships among these fastidious agents. Since the studies noted in the previous annual report, negative stain (phosphotungstic acid) electron microscopy was used in individual experiments as support for other studies such as examining stool suspensions prepared from individuals in an outbreak of gastroenteritis at Johns Hopkins University, examining harvests from tissue cultures inoculated with porcine norovirus positive stools, examining tissue culture preparations for murine norovirus particles, examining cesium chloride (CsCl) gradient fractions for murine norovirus particles, examining CsCl gradient fractions for feline calicivirus-like particles, examining a brain homogenate prepared from a mouse infected with murine norovirus, study by IEM of putative feline calicivirus virus-like particles, examination of a tissue culture harvest of a putative picorna-like virus from a CsCl gradient for virus particles, examining a CsCl gradient for rotavirus particles, examining sucrose gradient purified preparations for metapneumovirus particles (preparation from Respiratory Viruses Section).